Cathode ray oscillograph control circuits



Oct. 22, 1940. H. P. KUEHNI CATHODE RAY OSCILLOGRAPH CONTROL CIRCUITS Filed Sept. 1938 nou'uuu Inven tor:

Hans F2 Kuehrw by W His zttorney ti v Patented Oct. 22, 1940 fazialzis,

" csrnopn RaYosomLoGaArnooN o CIRCUITS PFK ueIinLrSchenectady, assigno rto Q J 4 -Application september zl,1'938, Seriall\lo231,035

General'Electric Company, a. corporation of N ew V,

My invention relates primarily-to 'the control circuits of cathode ray oscillographs-and in particular to the cathode ray beam initiating circuit and its object is to provide an impulse circuit particularly-adapted for the exact and positive control of the starting and stopping of the cath ode beam of high voltage 'oscillographs. The control scheme of my invention may be used whether the initiation of the cathode ray beam is'to be subject to manual or automatic control and whether the initiatingcurrent-signal be posreference is made in the following description to the accompanying drawing in which'Fig. 1 rep resents my improved apparatus, and associated circuit connection as applied for controlling the cathode of a cathoderayoscillograph and Fig.2 represents voltage time curves which will be referred to in explaining certain'operating features of my invention. Certain features of this invention and the general purpose of the 'oscillograph to which the invention is applicable are referred to -in an-article entitled A 'New' High-Speed Cathode-Ray Oscillograph, by Hi'PuKueh-niand Simon Ramo, pp." -721-727, Electrical Engineering, June 1937.'- Transactionsv of the American Institute of Electrical Engineers, vol. 56, 1937.

- In Fig.1 of the drawing there is represented a cathode-ray oscillograph'tube l0, having an inner anodeco-ating H in-the bell portion,la' hot cathode'IL-a beam intensity control or -modulating grid 13, an accelerating electrode I 4, --a

' focusing electrode [5, a pair of sweep plates I6 and a pairof deflection plates H. The invention to be described concerns a system for releasing, by the removal of the control grid bias, the oathode-raybeam between thecathode l2 and anode II at the instant desired to study or record surge voltages impressed between the deflection plates and to quickly .cut off the cathode-ray beamafter'la brief interval in order that'it will not cause fogging of the record that'mayhave been obtained.- 1 v "I In order to avoid confusion circuits such as the :circuits to the deflectionplate 116 and 11 which are not involved in the inventionhave been omitted from the drawing. The circuits to the auxiliary electrodes-l3, l4 and 15 while not.

' directly involved in the inventionare shown since the o ommitte suy dl th same source as-the cathode tub e excitatio n volt age =As an aid in explaining-the inventionandin order to give a practicable example, certain values of voltages, resistances, capacitances, etc., will be given in-the description which folloWs;-but it is to'be understood that the invention is-not to be limited to anyparticular combination of circuit voltages or circuit constants.

The circuit apparatus represented in Fig.- 1

- may beroughly grouped into three parts as fol:

lows; first,apparatus for supplying various op crating voltages shown in the right-hand portion' of the drawing; second, the cathode beam initiating unit having the impulse storage condenser l8 and a clischarge'circuitincluding the three-electrode vacuum tube l9 shown in the central left portion of the fieureyand third, the initiating unit shown at: the bottom of. thefigure, the" purpose of which is to initiate the operationof tube l9 when a signal comes in on the terminals 20, regardless of the polarity of the incoming signal.

The voltage supply apparatus comprises a transformer 2|, the secondary of which is con nected to supply a high direct-current'voltage across a number of series connected condensers 22, 23 and 24 through rectifier tubes 25' and 26'. The arrangement is such that the-rectified voltage is doubled. The condensers are shunted by a high bleeder resistance 28 which is:grounded at 29 and has'various taps'from which various.

values of supply voltage are taken ofi to" supply the oscillograph circuits as shown; If we takethegrounded point 29 as a zero voltage reference point, the polarity and value of the voltagesiat the other resistance tap may be as markedthereon; Thus, tap 3B'has a potential of 15;000'vo1ts and tap'3l has a-positivepotential of 1200Ivolts'.

The tap tocontrol gridv I3 is adjustable along one of the resistance sections 32 andthis section is shunted by a condenser in orderthat the beamv intensity may be stabilized and adjusted. The focusing electrode 15 has an auxiliary adjustable source of direct-current supplypotential consist,- ing of the transformer 33 andthe rectifier circuit 34. This adjustable voltage; however, is pro-per relation to the other electrode voltages-by reason of its being tiedtoresistance 28 at .the ,-7500 volt point.-

Theaccelerating electrode I l is connected to resistance 28 -at;the -14,600, voltage point, and

thus the auxiliary electrodesl3, l4 and 15 are supplied atproper potentials in relation .to the .e t q e' andan e; j

It will be noted that the anode I l is connected to ground and is thus at zero potential. The cathode I2 is connected to the 14,300 volt point through a 5 megohm condenser changing resistance 35 and condenser discharge resistance 36. Thus the cathode I2 has a potential of more than 600 volt positive with respect to the control grid I3, being equivalent to a 600 volt negative grid bias. Under this condition the cathode beam remains shut off. Variable condenser I8 is connected between the -14,300 and 4,900 volt points through its charging resistance 35 and as a result condenser I8 will be charged. The purpose of tube I9 is to discharge condenser I8 through discharge resistance 36 at desired times and in effect connect the 14,900 voltage point to the cathode through tube I9. When this happens and the discharge current through tube I9 is of sufficient magnitude, the negative grid bias is substantially removed between cathode I2 and control grid I3 initiating and maintaining the cathode ray beam. The discharge of condenser I8, however, is ordinarily prevented because the grid of tube I9 is normally held at a negative potential with respect to its filament cathode because the grid is connected to the 15,000 volt point of resistance 28 through a one megohm resistance 31. At times, however, when the cathode beam is to be initiated, the grid bias of tube I9 is made positive through the connection 38 as will be explained later, and at such times tube I9 becomes conducting to discharge condenser I8 and initiates the beam of the cathode-ray oscillograph I0. It is not only desirable that the cathode-ray beam be initiated positively and quickly at the desired instant, but also that it be maintained at high intensity for a short time and then quickly and positively cut oil. A high intensity beam is essential for high speed recording. It is necessary that it be of short duration because otherwise the tube will be damaged. To obtain high intensity and short duration of the cathode ray beam I employ high rate of discharge of condenser I8 for a short duration and a-positive cut-ofi while there yet remains an appreciable charge on condenser I8 and an appreciable voltage across it, for example, 100 volts. To this end I employ a discharge resistor at 36 which has a negative impedance current characteristic, for example, a resistor which transmits about 1 ampere at 1000 volts and .001 ampere at 100 volts. Resistance devices having such characteristics may be built in accordance with the teachings of United States Patent 1,822,742, Sept. 8, 1931, to McEachron. When the discharge current through tube I9 drops to the critical low value of about .001 ampere, the voltage across tube I9 is no longer suflicient to maintain the discharge against the negative bias of its grid which by this time is substantially restored to its original value.

The operation of tube I9 and its circuit may be made clearer by reference to Fig. 2 where the ordinates of the curve represent the voltage across tube I9 and the abscissa represents time. At the beginning of the curve it is assumed that condenser I8 is charged and tube I9 is not conducting. Thus A represents the voltage at the filament of tube I9 and B the voltage at the plate thereof. At point C an impulse comes in over line 38 and biases the grid of IQ so as to cause the tube to fire. It becomes conducting to discharge the charge on condenser I8 through resistance 36 and the voltage at the plate terminal immediately goes to value D and from time D to E- this discharge circuit is maintained.

During this time the negative voltage on cathode I2 is increased from about l4,300 volts to about -l4-.,900 volts less the drop in tube I9 and stabilizing resistor 39 and the cathode ray beam is established and maintained at high intensity. At point E it is assumed that the discharge current through resistor 36 has dropped to approxi mately .001 ampere and this rate of discharge is no longer able to maintain the tube path open against the negative bias of its grid and the tube becomes non-conducti g, whereupon the voltage across this tube immediately rises to F which is now the voltage remaining across condenser I8. Voltage F applied to cathode I2 reduces its negative voltage with respect to its anode II to a point where the'beam is no longer maintained and the beam is thus extinguished.

Had I used an ordinary resistance in place of resistor 36, the voltage applied to cathode I2 instead of changing abruptly at cut-off would change gradually as indicated by dotted line H and the point of cut-off of tube I9 and of the cathode beam of the oscillograph tube would have been delayed and the time of cut-off would have been rather indefinite. The cathode-ray beam instead of being cut off quickly while at good intensity, would have died out gradually with the likelihood of fogging any record obtained. After cut-off at point E, the charge on condenser I8 is gradually restored to its former value at about point G.

The impulse required for initiating the tube I9 is derived from the initiating unit shown at the bottom of Fig. 1. This unit consists of two three-- electrode vacuum tubes 40 and M connected in parallel. The two grids of these tubes are connected through a 20,000 ohm resistance 42, the mid tap of which is connected tov ground through a 1 mi. condenser 43 and resistance 44. The upper or grid terminal of this capacitor receives from a suitable source, which may conveniently be the source supplying resistance 28, a negative bias voltage of sufiicient magnitude toprevent these tubes from firing in the idling state of the apparatus. Thus the grids are connected through circuit 45 including a 5 megohm resistance to the 200 volt tap of resistor 28 while the filaments of these tubes are at the zero ground potential. The plates are connected to the +1200 volt point through a 5 megohm resistor 48 and to ground through a condenser 49. The grid of each tube 40 and 4| is connected to initiating or signaling terminals 20 through 1 mi. capacitors 46 and 41 and a balancing transformer. Terminals 20 may be connected to receive impulse signals from a transmission line Where the cathode ray oscillograph is used to record such surges automatically or from another device or circuit such as a manual switch.

If a voltage be applied to terminals 20, a current flows through resistance 42 in one direction or the other depending on the polarity of the initial voltage. The voltage drop across the resistance 42 produces a rise in potential on the grid of one tube and a lowering of potential on the grid of the other tube. The rise in potential on the one grid causes that particular tube to fire and the capacitor 49 which was intially charged to say 1200 volts discharges through the resistor 44 which may be of 100 ohms. Then there appears across the resistor 44 a voltage impulse of very steep wave front positive with respect to the grid of tube I9 to which it is transmitted through the coupling condenser 50 and wire 38.

The principal feature of this initiating unit is that it responds to A. C. or 'D. C. impulse Volt 7 ages and it is not necessary to watch or take into; accountthe polarity of the voltage source which is applied to the terminals 20. This initiating circuit and its desirable featuresis not restricted in its use to cathode ray oscillographs but can be er 49 through a solenoid indicated at 5|;

What I claim as new and desire to secure by Letters Patent of the United States is In combination with a cathode ray tube having a control gridja cathode and anode,'apparatus for providing an ignition voltage between said cathodeand anode comprising a voltage divider supplied bydirect current, connections between said voltagedivider, control grid, cathode and anode for impressing an ignition voltage between the cathode and an ode,- the cathode voltage bedenser terminal and cathode, means for momening the cathode-ray beam, and I a resistance through which the discharge current of said con denser flows, which resistance has a negative im- In ray beam instantly interrupted.

ing negative with respect to the anode voltage r... and a voltage on the control grid, negative with* respect to the cathode for preventing ignition of the tube, a condenser which is connected to be charged from said voltage divider between'the point thereof connected to the cathode and a v more negative pointya three-electrode vacuum I tube connected betweensaid more negative contarily rendering said vacuum tube conducting to discharge said condenserv to reduce thenegative grid bias with respect to thecathode and initiatpedance current characteristic whereby thefdischarge of, said condenser is abruptly interrupted beIore it is completely discharged whereby the negative grid bias is restored and the cathode" HANS P. 

